(1) Field of the Invention
The present invention relates to a cordierite honeycomb-structural catalyst carrier, and more particularly relates to a low-thermal expansion honeycomb-structural body having excellent thermal shock resistance, coatability, that is, coating capability of a high specific surface area material and a catalyst, and further having excellent thermal shock resistance after coating, which body is used as a carrier for a catalyst for purifying exhaust gas of an automobile, and a method for producing the cordierite honeycomb-structural body.
(2) Related Art Statement
There have recently been increasingly demanded materials having excellent heat resistance and thermal shock resistance corresponding to the progress of industrial technique. Particularly, the thermal shock resistance is one of the important properties demanded in ceramic honeycomb catalyst carriers to be used in purifying apparatuses for automobile exhaust gas. The ceramic honeycomb catalyst carrier to be used in a purifying apparatus for automobile exhaust gas is exposed to vigorous heat generation caused by a catalytic reaction of uncombusted hydrocarbons with carbon monoxide contained in the exhaust gas and to temperature variations by the rapid heating and rapid cooling at the start and stop of the automobile engine. Therefore the ceramic honeycomb catalyst carrier must have a thermal shock resistance high enough to resist the thermal stress caused by the temperature difference generated in the honeycomb-structural body. Particularly, the ceramic honeycomb catalyst carrier must have high thermal shock resistance in order not to be adversely influenced by the arrangement of the purifying apparatus near the engine for the purpose of increasing the catalytic activity, and by the high speed running of a car.
The thermal shock resistance is expressed by the tolerable temperature difference in the rapid heating or in the rapid cooling, and it has been known that this tolerable temperature difference is in inverse proportion to the coefficient of thermal expansion of a honeycomb-structural body among various properties thereof. That is, as a ceramic honeycomb-structural body has a smaller coefficient of thermal expansion, the honeycomb-structural body has a larger tolerable temperature difference, and that, in the honeycomb-structural body, its coefficient of thermal expansion in a direction (FIG. 4, axis B direction) perpendicular to the flow passage has a particularly high influence upon the tolerable temperature difference.
It has hitherto been known that cordierite ceramics are low in thermal expansion. For example, U.S. Pat. No. 3,885,977 (Japanese Patent Laid-open Application No. 50-75,611) discloses an oriented cordierite ceramic having a coefficient of thermal expansion of lower than 11.times.10.sup.-7 /.degree.C. within the temperature range of 25.degree.-1,000.degree. C. in at least one direction. U.S. Pat. No. 3,885,977 discloses a planar orientation of cordierite due to plating clay or delaminated clay of kaolin or the like, and discloses a composition which uses silica as a raw material and has a low coefficient of thermal expansion of 0.56.times.10.sup.-6 /.degree.C. within the temperature range of 25.degree.-1,000.degree. C.
This silica type honeycomb-structural body disclosed in U.S. Pat. No. 3,885,977 has a coefficient of thermal expansion of (1.01-1.08).times.10.sup.-6 /.degree.C. in a direction (in the axis B direction in FIG. 4) perpendicular to the flow passage of the honeycomb structure, which is higher than the coefficient of thermal expansion of (0.62-0.78).times.10.sup.-6 /.degree.C. in the flow passage direction (in the axis A direction in FIG. 4) of the honeycomb structure as disclosed in its working examples. Further, in this U.S. patent, the coefficient of thermal expansion in the axis B direction, which influences substantially the thermal shock resistance of the honeycomb-structural body, cannot be satisfactorily decreased.
U.S. Pat. No. 3,950,175 (Japanese Patent Laid-open Application No. 50-75,612) discloses that a cordierite based porous ceramic having open pores, at least 20% of which have a diameter larger than 10 .mu.m, can be obtained by replacing, partly or wholly, talc or clay contained in the starting materials by pyrophyllite, kyanite, quartz, silica, such as fused silica, or a silica-alumina source material.
Although this U.S. Pat. No. 3,950,175 discloses a cordierite based porous ceramic honeycomb-structural body produced by using fused silica as a silica raw material and having a large number of large pores having a diameter of larger than 10 .mu.m, this U.S. patent does not disclose the production of a low-thermal expansion cordierite honeycomb-structural body. Further, this U.S. patent, a cordierite honeycomb-structural body having a low coefficient of thermal expansion in its axis B direction cannot be obtained.
Japanese Patent Application Publication No. 57-28,390 discloses that the use of talc having an average particle size of 5-150 .mu.m can produce a ceramic honeycomb-structural body having a low thermal expansion of not higher than 1.6.times.10.sup.-6 /.degree.C. within the temperature range of 25-1,000.degree. C. However, this Japanese Patent application publication does not disclose at all a composition which gives a ceramic honeycomb-structural body having a low thermal expansion of lower than 0.9.times.10.sup.-6 /.degree.C. within the temperature range of 25.degree.-1,000.degree. C., and a ceramic honeycomb-structural body having satisfactorily low expansion in both of its axis A and axis B directions cannot be obtained.
The inventors have disclosed in U.S. Pat. No. 4,772,580 that finely divided talc having a particle size of not larger than 5 .mu.m is used as a base material, and is used in combination with high-purity non-crystalline silica and finely divided alumina in order to produce a compact ceramic honeycomb-structural body having a porosity of not higher than 30%. However, a cordierite honeycomb-structural body having a low thermal expansion lower than 0.3.times.10.sup.-6 /.degree.C. within the temperature range of 40.degree.-800.degree. C. can not readily be obtained by the method of this U.S. patent application.
The inventors have succeeded in the production of a cordierite honeycomb-structural body having a porosity of higher than 30% but not higher than 42% and having low thermal expansion in its axis A direction and in its axis B direction.
In a ceramic honeycomb-structural body to be used as a catalyst carrier, not only its low thermal expansion property, but also its coatability with a high specific surface area material and a catalyst are important properties.
It has been necessary to coat .gamma.-alumina having a large specific surface area on the surface of walls of a cordierite honeycomb-structural body in order to support a catalyst substance. The coatability with a high specific area material and a catalyst is one of the important properties demanded from a cordierite honeycomb-structural body to be used as a catalyst carrier. The cordierite honeycomb-structural body to be used as a catalyst carrier also must have a certain degree of porosity in order to satisfy the above described demands and to carry out the mass production of cordierite honeycomb catalysts.
Honeycomb catalysts are produced by coating the surface of a cordierite honeycomb-structural body having a low thermal expansion with a high specific surface area material, such as activated alumina or the like, having a thermal expansion remarkably higher than that of the cordierite honeycomb-structural body. Therefore, when the thermal expansion of a cordierite honeycomb-structural body is merely decreased, the thermal shock resistance of the resulting honeycomb catalyst can not be improved. That is, such a technique is demanded that the increasing of thermal expansion of the cordierite honeycomb-structural body due to the coating its surface with a high specific surface area material and a catalyst is suppressed as small as possible.
Japanese Patent Application Publication No. 51-44,913 discloses that the coatability of a ceramic honeycomb-structural body is improved by a method, wherein a ceramic powder is deposited onto the surface of the thin walls of the honeycomb-structural body, and the ceramic powder-deposited honeycomb-structural body is fired to form a surface layer having pores in a volume of not less than 0.1 cm.sup.3 /g, each pore having a diameter of not smaller than 5 .mu.m.
However, in the method of this Japanese Patent Application Publication No. 51-44,913, the deposition step of ceramic powder to the honeycomb-structural body is required, and the resulting honeycomb catalyst carrier is expensive. Moreover, a honeycomb catalyst carrier having pores having a diameter of 0.5-5 .mu.m defined in the present invention is difficult to produce.
Japanese Patent Laid-open Application No. 58-14,950 discloses that, when activated alumina of a high specific surface area material is to be coated onto a cordierite honeycomb-structural body, an organic substance, such as methyl cellulose or the like, is precoated onto the honeycomb-structural body so that the activated alumina can not enter into micro-cracks formed in the cordierite honeycomb-structural body, whereby the thermal shock resistance of a cordierite honeycomb catalyst is improved. However, the adhesion of high specific surface area material, such as activated alumina, to the cordierite honeycomb-structural body is deteriorated during the use of the resulting honeycomb catalyst, and the coated layer is apt to peel off. Moreover, working steps required for the coating are increased, and the resulting cordierite honeycomb catalyst is very expensive.
The inventors have succeeded in the production of a cordierite honeycomb-structural body to be used as a catalyst carrier, having excellent thermal shock resistance after coating with a high specific surface area material and a catalyst, by limiting not only the porosity and coefficient of thermal expansion of the body but also the distribution of the diameters of pores in the body.